The invention is based on a compact low-pressure discharge lamp in accordance with the preamble of claim 1. This is in particular a compact low-pressure discharge lamp having a discharge vessel comprising at least four straight, parallel tubes which are arranged in a polygon and, at or close to the ends of the straight tubes, are connected by transverse connections to form a single continuous discharge path which is closed off in a gastight manner.
The compact low-pressure discharge lamps having a discharge vessel which comprises four or more straight, parallel tubes and is assembled by means of transverse connections, depending on the length and the diameter of the discharge vessel and on the internal diameter of the transverse connections, often require very high voltages to be fired reliably.
It is known from U.S. Pat. No. 6,064,152 to introduce a hollow cylinder made from electrically conductive material in the form of a metal foil into the hollow interior formed by the straight, parallel tubes of the discharge vessel. This allows the firing voltage of the lamp to be reduced considerably.
However, a drawback is that a metal cylinder of this type absorbs a large proportion of the light which is radiated inward from the discharge vessel, and this light is therefore lost. Moreover, a metal cylinder of this type changes the temperature balance of the lamp. For example, the metal cylinder leads to an increase in the cold-spot temperature, which in turn leads to a shift in the radiation maximum toward lower ambient temperatures.
Therefore, it is an object of the invention to provide a compact low-pressure discharge lamp with a reduced firing voltage which allows the light which is radiated into the cavity formed by the straight tubes and the transverse connections of the discharge vessel to pass substantially without being impeded.
In a compact low-pressure discharge lamp having the features of the preamble of claim 1, this object is achieved by the features of the characterizing part of claim 1. Particularly advantageous configurations are listed in the dependent subclaims.
A metal coil spring means that the space inside the straight tubes of the discharge vessel remains substantially clear. Consequently, most of the radiation which is emitted into the central cavity between the straight tubes of the discharge vessel can pass without being impeded or can pass back out after having been reflected one or more times from the discharge vessel walls. Moreover, the temperature balance of the low-pressure discharge lamp is only affected to an insignificant extent.
In a preferred embodiment, the coil spring may have a reflective coating. This enables the radiation which is emitted into the center of the discharge vessel and impinges on the coil spring to be partially radiated back outward, so that the radiation loss caused by the introduction of the coil spring is reduced further. The coating preferably has a reflectivity which corresponds to that of the phosphor-coated discharge vessel.
The coil spring advantageously consists of wire, a wire diameter of between 0.05 and 1 mm being selected, depending on the extent to which it is necessary to reduce the firing voltage.
The extent to which the firing voltage is reduced can be set by means of the number of turns of the coil spring which bear against the walls of the straight tubes of the discharge vessel. In this connection, it is merely necessary to match the diameter of the number of turns of the coil spring which are to bear against the discharge vessel to the diameter of the cavity.
The pitch factor PF, i.e. the ratio of the distance between two adjacent wire turns to the diameter of the wire, determines the number of wire turns which a coil spring of a defined length possesses. The number of turns of the coil spring can in turn be used to define the extent to which the firing voltage is reduced. In a preferred embodiment, therefore, the coil spring has a pitch factor PF of 1.5 less than PF less than 70.
To securely hold the coil spring in the cavity between the straight tubes, the coil spring, in the stress-free state, preferably has a starting length which is between one and five times the distance between the transverse connections of the discharge vessel at the end remote from the cap housing and that end of the cap housing which faces the discharge vessel. Moreover, the last turn or last turns of the coil spring at the end remote from the cap housing preferably has or have a diameter which is such that they bear against all the walls of the straight tubes. This allows the coil spring to be clamped between the cap housing and that outer wall of the transverse connections remote from the cap housing which faces the cap housing, so that it is held securely between the parts of the discharge vessel.